With an advancement of digitalization in an information-oriented society, there is an increasing demand for a higher density recording/reproduction on a writable medium.
In order to increase a recording capacity in a so-called writable optical recording medium, various medium structures have been tried. Also, in order to realize a higher density recording/reproduction, various measures have been taken such as i) shortening the wavelength of a laser beam for recording and reproduction, ii) enlarging the NA (numerical aperture) of an object lens with which light is focused onto the optical information recording medium, iii) providing a plurality of recording layers to the optical information recording medium, and iv) forming a mask layer in the optical information recording medium for substantially reducing the diameter of a laser light spot.
For example, Japanese Unexamined Patent Publication Nos. HEI 5(1993)-12673, HEI 5(1993)-12715 and 2001-35012 have proposed the technique of forming a mask layer in the optical information recording medium for substantially reducing the diameter of a laser light spot.
According to this technique, in an optical information recording medium, the mask layer is formed at a location nearer to a side from which light is incident than a recording film. The mask layer is usually made of a thermochromic material or of a phase change material. Upon irradiation of the mask layer with a light beam for reading, a light spot is formed on the mask layer, and as the temperature rises at a central part in the light spot, the mask layer changes optically or thermally to partially have its color erased and to partially become light-transmissive so that the size of the light spot is substantially reduced, while with no incident light or with only weak incident light, the mask layer has a small transmittance. That is, the mask layer allows light to pass through only a part thereof where light intensity is high so that recording/reproduction of small pits is achieved. In other words, the diameter of a light spot can substantially be reduced and thereby a high density recording/reproduction on the optical information recording medium can be achieved.
By being heated above a predetermined temperature, the mask layer melts to exhibit a masking effect. In a melt state, the mask layer has a high flowability and has a composition and a shape changed from original ones. Therefore, repeated recording/reproduction on the optical information recording medium causes the change in the composition and the shape of the mask layer, and thereby a masking effect gradually diminishes, and after about several thousands of repeated recording/reproduction, the masking effect substantially vanishes.
Further, in the optical information recording medium, the mask layer formed at a side near to an incident light side absorbs not a small portion of the entire amount of light that otherwise reaches the recording layer. As a result, a reduction in recording sensitivity and a rise in reproduction noise are caused, whereby a high signal quality cannot be obtained.
Also, since the transmittance or reflectance of the mask layer is changed by a change in the composition of the mask layer or a dye reaction thereof, the transmittance or reflectance of the mask layer cannot be changed to a greater degree, even if the mask layer is heated so that its temperature is raised by 100° C., than a degree allowed by the physical properties of a substance used for the mask layer. Thus, there is a limitation on a signal quality obtained.